This invention relates generally to methods and apparatus for the recovery of vapors of low volatile fluids such as liquid hydrocarbons or liquid chemicals whose vapors may pollute and contaminate the air in a confined space or the atmosphere generally and more particularly to a system and apparatus for the the recovery of such vapors by condensation of the vapors at atmospheric pressure through specially designed non-contacting heat exchange apparatus and the supporting cooling systems therefor.
Since the passage of the Clean Air Act, the Congress of the United States has required that all persons or organizations handling hydrocarbons or chemicals whose vapors may pollute the air must install means to recover and prevent the contamination of the air by such vapors.
For example such vapors are generated and displaced into the atmosphere when all types of tanks are filled with liquid hydrocarbons or liquid chemicals. Such tanks may be large storage tanks, railroad car tanks, truck tanks, underground storage tanks for gasoline stations and fuel tanks on trucks, buses and automobiles. When these various types of tanks are filled with liquid hydrocarbons or liquid chemicals vapors escape into the atmosphere and as is well known such vapors become a source of smog which under certain ambient conditions produce dangerous fog conditions and so pollute the atmosphere that they produce dangerous environmental health hazards for human beings.
There are at least four methods with corresponding apparatus for recovering or eliminating these detrimental vapors. These may generally be summarized as burning, adsorption, absorption and condensation. Each of these methods and the associated apparatus have advantages and disadvantages as is well known and understood by those skilled in the art.
The present invention treats with the condensation techniques wherein the generated vapors are condensed at atmospheric pressure by non-containing heat exchange relation i.e. by condensation of the vapors at atmospheric pressures on cold surfaces.
This condensation method and apparatus in accordance with the present invention has a low operating cost and does not waste valuable and expensive hydrocarbon fuels such as methane, ethane and propane. Methane, ethane and propane are used in the above described method of burning the vapors and therefor wasted.
The condensation method also has a moderate capital cost and low maintanence costs which distinguishes it from the adsorption methods and apparatus where there is a high capital cost for the apparatus and high operating costs because of replacement of the activated charcoal materials used to adsorb the vapors being recovered. It does not have the compression requirements, high maintainence costs and small net recovery of condensed vapor as exist with respect to the absorption methods and apparatus.
The condensation method and apparatus of the present invention has a sufficiently high rate of recovery of hydrocarbons or chemicals that the value thereof may be sufficient to pay for the capital cost of the equipment, the operating and the maintanence cost thereof.
Various types of absorption systems are shown in U.S. Pat. Nos. 2,849,150, 3,771,317, 3,815,327 and 3,830,074.
Vapor recovery devices which utilize condensing techniques of the non-contacting type are shown in U.S. Pat. Nos. 2,379,215 and 3,648,436.
In U.S. Pat. No. 3,714,790 the vapors to be recovered are compressed and then past in direct contact with a cooling liquid of the same low volatile substance from which the vapors emanated, to condence the vapors and to recover the same.
The present invention is directed to the recovery of hydrocarbon vapors and more particularly those hydrocarbon vapors being emitted during the handling of fuels such as gasoline, diesel fuel, jet engine fuels etc., which predominately emit vapors such as butane, iso-butane, pentane, iso-pentane and heavier hydrocarbons in access of six carbon linkages.
It is known that the percentage of hydrocarbon vapors in the mixture of vapors vented from a given tank or like storage facility as it is filled ranges up to 65% of the vented mixture depending upon the ambient temperature, type of fuel, reed vapor pressure of the fuel, type of tank loading (splash or bottom loading) and the degree of agitation of the tank. The remaining portion of the vented mixture will include, water vapor and non-condensible gases.
Water vapor presents a problem in non-contacting condensing apparatus because the condensing surfaces therein disposed for heat exchange relation with the predominant and recoverable vapors above enumerated must be maintained, at atmospheric pressure, in a temperature range of -70.degree. F. to -110.degree. F., in order to condense and efficiently recover these fractions of the vented mixture escaping or leaving a given tank.
Because the water vapor freezes out of such mixtures at 32.degree. F., it constitutes a difficult component of the vented mixture in that at these temperature ranges for efficient recovery, frost or icing of the non-contacting heat exchange surfaces in the flow passages of the condensing apparatus; and more particularly on the finned surfaces which are generally provided such heat exchange surfaces to increase the area of heat exchange; will act either to stop the flow of the vented mixture through the flow passages in the condensing apparatus or will produce a back pressure in the flow passages or the condensing apparatus which will cause the relief valves on the tanks being filled or emptied to open and thus cause recoverable vapors to be expelled to the atmosphere.
The present invention overcomes this problem by providing an improved vapor condenser wherein the non-contacting heat exchange surfaces the flow passages therethrough are designed so that frost or icing buildup thereon is prevented from blocking off the flow of the vented mixture through the heat exchange members in the flow passages of the vapor condenser, and the flow passages have a U-shaped design to act as a thermal trap for preventing convection currents of ambient air from entering the vapor condenser at off cycle or low load conditions as the moisture therein would quickly plug the heat exchange surfaces or fins with frost and ice.
Further the system includes means to defrost the vapor condenser so as to control and prevent frost or icing of the heat exchange surfaces during the normal course of operation of the system.